Inkjet inks used for inkjet printing system are generally classified into aqueous inks and non-aqueous inks.
Aqueous inks comprise water as a solvent and thus easily provide printing images high in density and quality when plain paper is used as a printing medium, because the solvent easily permeates into the printing medium so that pigments are easily kept on the surface of the printing medium. On the other hand, aqueous inks are disadvantageous in that they easily cause the printing medium to generate curl and cockling, thereby affecting conveyance of the printing medium and producing a bad effect on high-speed inkjet printing.
Non-aqueous inks are generally classified into solvent inks composed mainly of a very-volatile organic compound as a solvent and oil inks composed mainly of a low-volatile organic compound as a solvent. Solvent inks are excellent in drying, but operating environment is limited because a large amount of solvents are volatilized. On the other hand, oil inks are excellent in permeability into printing media and drying when plain paper is used as printing media, and also volatize less solvents than aqueous inks and solvent inks. Thus, oil inks are advantageous in that clogging in ink nozzles is unlikely to occur, thereby reducing frequency of cleaning of ink nozzles, and thus are suitable for high-speed printing, particularly high-speed line head inkjet printing system.
However, oil inks are defective in that they are poor in separation of the coloring material and the solvent on printing media, and particularly when plain paper is used as printing media, both the coloring material and the solvent are easy to permeate into a gap between fibers of printing media, thereby causing deterioration of quality of printing images due to decrease of printing density, increase of strike through and bleeding of printing dots.
Conventionally, as a method for staying a coloring material and a solvent together on a surface of paper without permeation, it has been proposed for a aqueous ink that a reactive substance is contained in an ink whilst a treatment solution is provided which contains a substance reactive with the above substance, and the treatment solution is ejected onto the ink, whereby both substances are reacted on the surface of paper to cause aggregation of coloring materials, thereby making it difficult for the coloring materials to permeate into the paper (refers to patent Document 1 and Patent Document 2). However, no method exhibiting a similar effect has been proposed for non-aqueous inks.
Also, there has been proposed a printing system in which an ink containing an ultraviolet or electron-beam curable organic compound as a solvent is used, and the ink is ejected onto the printing medium and then irradiated with an ultraviolet beam or heated so as to render the ink to cure and stay on the surface of paper. However, it requires an irradiation device and a heating device, thereby making the device to be large-scaled and increasing electricity consumption.
[Patent Document 1] JP-A-H8-281930
[Patent Document 2] JP-A-2000-198263